#include<bits/stdc++.h>
using namespace std;
#pragma GCC optimize("O3")


#define rep(i,n) for(ll i=0;i<n;i++)
#define repl(i,l,r) for(ll i=(l);i<(r);i++)
#define per(i,n) for(ll i=(n)-1;i>=0;i--)
#define perl(i,r,l) for(ll i=r-1;i>=l;i--)
#define fi first
#define se second
#define ins insert
#define pqueue(x) priority_queue<x,vector<x>,greater<x>>
#define all(x) (x).begin(),(x).end()
#define CST(x) cout<<fixed<<setprecision(x)
#define rev(x) reverse(x);
using ll=long long;
using vl=vector<ll>;
using vvl=vector<vector<ll>>;
using pl=pair<ll,ll>;
using vpl=vector<pl>;
using vvpl=vector<vpl>;
const ll MOD=1000000007;
const ll MOD9=998244353;
const int inf=1e9+10;
const ll INF=4e18;
const ll dy[9]={1,0,-1,0,1,1,-1,-1,0};
const ll dx[9]={0,1,0,-1,1,-1,1,-1,0};
template <typename T> inline bool chmax(T &a, T b) {
  return ((a < b) ? (a = b, true) : (false));
}
template <typename T> inline bool chmin(T &a, T b) {
  return ((a > b) ? (a = b, true) : (false));
}

int main(){
  ll n,s,id;cin >> n >>  s >> id;
  vl v(n);rep(i,n)cin >> v[i];
  ll f=v[id];v[id]=INF;
  sort(all(v));v.pop_back();
  rev(all(v));
  f+=s*100;
  vl score(n-1);
  rep(i,n-1){
    score[i]=s*50*(1+10/(2*(i+1)+8));
  }
  rev(all(score));
  double ans=1;
  rep(i,n-1){
    ll need=f-v[i];
    if(need<0)ans=0;
    ll s=upper_bound(all(score),need)-score.begin();
    s-=i;
    if(s<=0)ans=0;
    ans*=double(s)/(n-1-i);
  }
  CST(10);
  cout << ans << endl;
}